Conventional optical transmission systems use optical fibers to carry large amounts of data over long distances from a transmit terminal to a receive terminal. Wavelength division multiplexing (WDM) has been explored as an approach for increasing the capacity of existing fiber optic networks. In a WDM system, plural optical signal channels are carried over a single optical fiber with each channel being assigned a particular wavelength. The maximum distance that the optical signals can be transmitted in the fiber without amplification or regeneration is limited by the loss and dispersion associated with the optical fiber. To transmit optical signals over long distances, the transmission systems may include a number of optical amplifiers periodically located along the fiber route from the transmit terminal to the receive terminal. Each amplifier boosts the weak received signal to compensate for the transmission losses, which occurred from the last amplifier. For example, optical channels in a WDM system are frequently transmitted over optical fibers that have relatively low loss at wavelengths within a range of about 1525 nm to 1580 nm. WDM optical signal channels at wavelengths within this low loss “window” can be transmitted over distances of approximately 50 km without significant attenuation. For distances beyond 50 km, however, optical amplifiers are required to compensate for optical fiber loss.
Optical amplifiers have been developed which include an optical fiber doped with a rare-earth element such as erbium. The erbium-doped fiber is “pumped” with light at a selected wavelength of either 1485 nm or 980 nm, which coincide with the absorption peaks of erbium. At the same time, a communication signal is passed through the doped fiber to provide amplification or gain at wavelengths within the low loss window of the optical fiber. However, erbium doped fiber amplifiers do not uniformly amplify light within the spectral region of 1525 to 1580 nm. For example, an optical channel at a wavelength of 1540 nm could be amplified 4 dB more than an optical channel at a wavelength of 1555 nm. While such a large variation in gain can be tolerated for a system with only one optical amplifier, it cannot be tolerated for a system with plural optical amplifiers or numerous, narrowly-spaced optical channels, particularly when the gain is sufficiently large to cause nonlinear propagation, which adversely affects transmission of the signal. In these environments, much of the pump power supplies energy for amplifying light at the high gain wavelengths rather than amplifying the low gain wavelengths. As a result, low gain wavelengths suffer excessive noise accumulation after propagating through several amplifiers.
Optical amplifiers with nominally wavelength-independent gain over a wide wavelength range are conventionally designed with a static filter having a wavelength-dependent loss that complements the amplifier wavelength-dependent gain. Wavelength dependent couplers, thin-film dielectric filters and fiber gratings are examples of three static filter technologies used in this application. These static equalizers can only provide a fixed amount of attenuation per channel and thus cannot correct for changes in the amplifier gain shape that results from a change in pump power or amplifier input power. This problem is conventionally corrected by inserting a Variable Optical Attenuator (VOA) to dynamically adjust the signal power in the amplifier. The VOA corrects for any discrepancy between amplifier gain and span loss, which ideally should be equal. This is necessary in practice because variation in fiber attenuation or span length alter the power level of the signals at the input to the amplifier.
Another phenomenon relating to the spectral gain profile of optical amplifiers is known as gain tilt, which is a particular problem for dynamically changing and/or reconfigurable dense wavelength division multiplexed communication links. Gain tilt arises when there are dynamic changes in operating conditions such as the input power and wavelengths of the transmitted channels. For example, when a channel is added or subtracted, thus changing the input power and spectrum of the optical signal, a gain fluctuation occurs in dependence on the channels' wavelength to effectively “tilt” the gain of the amplifier. An optical amplifier with Automatic Gain Control (AGC) programmed to maintain constant gain can be used to correct the aforementioned dynamic instability by decreasing or increasing the amplifier pump power when dropping or adding channels, respectively. However, improved amplifier performance is possible if AGC is implemented in a Variable Gain Amplifier (VGA) such that the amplifier gain can be adjusted to exactly offset the loss of the preceding span, without adding VOA loss to the low gain wavelengths. Such a VGA requires a filter that can dynamically adjust the wavelength-dependent loss to correct for gain tilt that arises from the change in pump power used to adjust the amplifier gain. Such a filter is typically described as a Dynamic Gain Equalizer (DGE), and it can also improve system performance by equalizing channel powers to correct for wavelength-dependent loss (WDL), polarization dependent loss (PDL), or even a laser transmitter that has an incorrect launch power. The net effect of adding a DGE is to achieve a system with better amplifier performance and greater uniformity of signal powers, which can enable more reliable transmission at higher data rates and/or over longer distances.
Dynamic gain equalizers can be used in optical transmission systems for purposes other than to compensate for nonuniformities in the gain of optical amplifiers. For example, the gain of an optical transmission system as actually deployed may not precisely match its design specifications. In such a case gain equalizers that provide a fixed gain profile and which are configured for the system's design specifications, as opposed to its actual specifications, will not necessarily result in gain equalization.
For the above reasons it is clear that there are significant advantages to employing a dynamic gain equalizer that compensates for dynamic fluctuations in the gain of an optically amplified system. While a number of technologies have been proposed to form a DGE as discrete elements, including heated waveguide arrays [U.S. Pat. No. 6,212,315], acoustooptic gratings [U.S. Pat. No. 6,021,237] and tunable fiber gratings [U.S. Pat. No. 6,151,157], none of the existing technologies have been extensively used to date in a deployed system because they suffer from a variety of shortcomings. These shortcomings include one or more of the following: high cost, high insertion loss, and excessively large physical size—sometimes a size that is larger than the amplifier itself.
Accordingly, it would be desirable to provide a dynamic gain equalizer for use in an optical transmission system that overcomes the aforementioned deficiencies.